278 



SIMPLE AND MULTIPLE ARCS. 



impression made on the free extremity of a instantly produces a con- 

 Fia 128. traction in the muscular fibre m, which the effe- 



rent branch e supplies. The whole force is at 

 once consumed, no portion of it remaining. 



The simple cellated nerve arc, Fig. 123, con- 

 sists of a centripetal fibre, <z, which, receiving 

 impressions on its free extremity, conveys them 

 to the vesicle v, from which the influence passes 

 forward along the centrifugal fibre 6, causing the 

 simple ceiiated arc. muscular fibre m, which the nerve supplies, to 



contract. An impression made at a therefore produces motion at m. 

 The action is purely automatic, and a part of the force is stored up or 

 remains in the vesicle. 



In the figures here given, the centripetal and centrifugal fibres are rep- 

 resented apart. In fact, however, they may be considered as bound to- 

 gether, for the sake of compactness, without there being any fusion or 

 coalescence of structure or functions. It is also to be understood that 

 the free extremity of the centripetal fibre is connected with some special 

 mechanism adapted to the influence it is to receive. Thus its axis cyl- 

 inder may be naked, or connected with a vesicle, or with an apparatus 

 for the reception of light, or sound, or heat, or pressure, &c. 



Multiple automatic nerve arcs arise from an arrangement of many such 

 simple arcs in succession longitudinally, as in Fig. 

 124, or it may be in a circular order. The former 

 case is presented in the articulata, the latter in the ra- 

 diata. Each symmetrical portion of the animal has 

 its own nervous arc, but as such symmetrical portions 

 are not destined to live an independent life, but to act 

 in unison with the others, a necessity arises for each 

 arc to be brought in relation and maintain a connec- 

 tion with the others, and this is done by extending 

 from ganglion to ganglion fibres of communication, <?, <?, which may here 

 Commissurai be called commissural fibres. So the circle of ganglia which 

 fibres. surrounds the mouth of the radiata is not a circular arrange- 



ment of isolated ganglia, but a ring of ganglia and commissures conjoint- 

 ly. Where the nervous system is planned symmetrically on the two 

 sides of the mesial plane, the ganglia are commissured across the plane 

 to insure a reciprocity of action. In the molluscs, whose organs of ani- 

 mal life show this bilateral symmetry, and which have three such gan- 

 glia, the cephalic, pedal, and parieto-splanchnic, each is commissured with 

 its colleague on the other side of the plane, or they are brought up to the 

 plane and juxtaposed, the commissure then disappearing, but their bi- 

 lobed aspect betraying their separate construction. The coalescence fre- 



Fig. 124. 



Multiple nerve arcs. 



